Meeting the growing global energy demand in a sustainable way can be regarded as the most significant scientific challenge in our era. In response to this challenge, significant technological innovations are required, which in turn require advances in basic science, especially studies on basic processes such as solar energy capture, conversion and storage. Researchers have made considerable efforts to develop photocatalytic decomposition of H2O into H2 and O2, conversion of CO2 and H2O to CH3OH and O2, and similar conversion for energy absorption. Such researches have received extensive attention in the chemical and material sciences. Another solar energy storage strategy which has been relatively unexplored to date is that high-energy metastable compounds are generated by using photochemistry, which, at a later time, are capable of releasing large amounts of heat through their thermal isomerization. Such a process of a photoisomerization reaction followed by reversion of isomerization to release heat can be repeated multiple times, which represents a closed cycle of storing the solar energy by using high-energy chemical bonds. This process offers unique attributes that make this cycle a useful hybrid solution for solar energy capture, storage and utilization. This process is one that photosensitive molecules are used to both convert and store solar energy using the same material. These molecules convert the solar energy into tight or rearranged chemical bonds and thereby have an energy storage space, i.e. the energy difference between the ground state and the metastable state. A parent photoisomer is converted to a high-energy metastable photoisomer by irradiation of light. The photoisomer is then converted to the original parent compound by exposure to a catalyst or by heating. During the reversion of conversion, the photoisomer releases the stored energy in a form of heat. These materials are used for solar energy capture and thus are referred to as solar thermal fuels, which operate in a closed cycle having a high energy density without degradation or divergence in an ideal state and can conveniently distribute the stored solar energy according to “heat demand”.
An azobenzene is a photoresponsive material having both cis and trans configurations, which meets the requirements of this method. Under the irradiation of ultraviolet light with a specific wavelength, the azobenzene in a trans configuration is converted into a cis configuration; and thereafter, under light or heat conditions, the cis configuration can be reverted to the trans configuration. There is an energy difference between the two configurations, in which transition from a trans configuration to a cis configuration can store energy, and in reverse, heat can be released. As a photothermal energy storage material, there are several factors that need to be optimized, including the energy storage density, solar spectrum matching (the ability of a molecule to absorb large bands of the solar spectrum), quantum yield of the light conversion, half-life of high-energy isomers, and height of an energy barrier to revert the conversion, etc. However, simultaneous optimization of these factors in a single molecule system is very challenging. The azobenzene has a good cycle performance and is therefore widely used in various optical switches; however, its application in photothermal storage is limited by its low energy density.